Filled paper is made by a process comprising providing a dilute aqueous suspension (termed a thinstock) of cellulosic fibres and filler, draining the thinstock suspension to form a sheet, and drying the sheet. It is desirable to retain as much as possible of the filler and fibre, including fibre fines, in the sheet and it is normal to add a retention aid to the thinstock in order to promote retention.
The thinstock is usually made by diluting with water (typically white water from the drainage stage) a more concentrated suspension of filler and cellulosic fibre. This more concentrated suspension is normally called the thickstock. The thickstock may be made merely by blending together the desired amounts of a single supply of fibre, a single supply of filler and water, or by blending several different supplies of fibre and/or filler and water.
Some of the feed to the thickstock can be recycled material, for instance deinked pulp, and if the recycled pulp contains filler this previously used filler will be incorporated into the thickstock. Often additional, previously unused, filler is incorporated into the thickstock or thinstock.
Polymers of a wide range of molecular weights can be used as retention aids, and it is also known to add a high molecular weight polymeric retention aid to the thinstock after incorporating a lower molecular weight polymeric coagulant into the thinstock or even the thickstock.
For instance it is known to treat unused filler with polymeric coagulant before adding that filler to the thickstock. The purpose of this coagulant addition is to coagulate the filler and thereby improve its retention. Unfortunately the process tends to result in the filler being less satisfactory (e.g. it gives less opacification) and so the addition of coagulant in this manner is not entirely satisfactory.
In many processes for making filled paper, a cationic, high molecular weight, retention aid is added to the thinstock formed from good quality pulp (of low cationic demand). In such processes, the addition of retention aid usually results in improved retention of both filler and fines.
In EP-A-17353 a relatively crude pulp, having high cationic demand, is treated with bentonite followed by substantially non-ionic polymeric retention aid. Although the suspension in this process is a substantially unfilled suspension, in AU-A-63977/86 a modification is described in which the suspension can be filled and in which bentonite is added to thickstock, the thickstock is then diluted to form thinstock, a relatively low molecular weight cationic polyelectrolyte is added to the thinstock, and a high molecular weight non-ionic retention aid is then added. Thus in this process, coagulant polymer is used, and it is added to the thinstock after the bentonite.
Processes such as those in EP 17353 and AU 63977/86 are satisfactory as regards the manufacture of paper from a suspension that has relatively high cationic demand and relatively low filler content, but tend to be rather unsatisfactory as regards filler retention when the suspension contains significant amounts of filler.
It would be desirable to be able to improve filler retention in paper-making processes such as those of EP 17353 and AU 63977/86.